A giant pond to help improve water quality

It might look like just a giant pond, but this project to reduce sediment and nutrients entering the Waihopai River and New River Estuary, is already proving successful.

Invercargill City Council capital projects technician Leonardo Ramirez says the sediment pond near the river at the bottom of Beatrice Street in Invercargill has been constructed to collect stormwater from drains within 82 hectares of the city.

The $500,000 project was completed late last year, after first being mooted by drainage manager Malcolm Loan in 2012.

The 24-metre by 200-metre pond holds approximately 5,000m3 of water. The water from stormwater drains has traditionally fed straight into the river, taking with it large volumes of sediment and any rubbish picked up along the way that wasn’t caught by the grates in the pumps.

Leonardo says they knew the water quality could be improved by lengthening the time the water spent getting to the river, allowing the sediment and rubbish time to settle and sink to the bottom.

“Ponds like this have been very successful in other regions in New Zealand, so we followed the Christchurch City Council’s guide quite closely.”

“Improving the water retention time is the simple key to improving the water quality at the outlet, the sunlight then has the opportunity to kill much of the bacteria and that has to be positive.”

Just a week after the pond was installed, Leonardo says results were already obvious, with sediment reduced by up to 50%. The pond is likely to need the sediment removed every 5-10 years, but will hopefully require little other maintenance.

For Leonardo, who moved from Colombia several years ago, being involved in such a large project with really constructive outcomes has been a fantastic experience.

“It was exciting to be part of such a positive project. Here we had a ditch and the water we were purging was just rubbish, but we’ve made big improvements and that’s great.”

pond2 — The new sediment pond near the bottom of Beatrice Street (above and background).

It’s not only the water that is benefitting from the pond, the environment around it has also seen significant improvement, with around 9,000 plants added to help support the area’s biodiversity and assimilate nutrients like nitrogen and phosphorus.

A community planting day was held to get the plants in the ground and this was the first real opportunity for the public to take a look at something that had been the subject of much public speculation about its purpose during construction.

Leonardo would like to see a second pond built, which he thinks would complement the first and improve the water quality more. He hopes this may be part of the longer term planning for the city's infrastructure.

You can read the full Envirosouth magazine online here.

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A giant pond to help improve water quality

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A giant pond to help improve water quality

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